While most people do shoot RAW and can tinker their white balance settings (WB) by GUI sliders from their software (such as Lightroom, and PhotoShop), sometimes, it’s beneficial to know how to do it from scratch as well.
Of course, it’ll be best to make sure your WB is accurate to begin with, but the world is not perfect, hence image editing software are made available.
Portraits, in general, require a certain level of color accuracy to make the skin of the subjects flattering. Light skinned subjects has a tendency to look too pink with wrong WB, while dark skinned people can be too orangey.
We’ll be using CMYK (cyan, magenta, yellow, black) to reference our skin tone correction. The premise is, light skin should roughly have a ratio of approximately Magenta + Yellow values = 5 times the Cyan value.
Before you panic about the equation, it’s really quite simple. If your Cyan value is 10%, your Magenta and Yellow value should add up to roughly 50%. Depending on the obvious color cast, either the Magenta or Yellow value will be higher.
It’s also worth noting that lighter skin tones will have a smaller ratio, usually a M+Y = 3x C, instead of 5x C… Conversely, dark skin tones will have a higher ratio.
These are just guideline figures as human skin tones vary greatly from race to race.
Here’s the original image with severe color cast from the tungsten lights in the room with the camera’s white balance at (auto) AWB setting.
1) Open your image and select your Color Sampler tool. I prefer to pick a larger sampling area (CS2 only allows up to 5×5, I think CS3 can go much higher).
Click on a non-reflective part of the face that you want to use as a reference point. I prefer the middle cheek, forehead, chin, or nose bridge in most cases. Here, I chose the area between the eyes.
I also sampled the background wall as I know that the wall is off-white with a hint of pink. If I can get that close to actual, then I know I’m close to correcting the skin as well.